I recall reading that the corals can reproduce both sexually and by cloning, with most in nature using cloning. The researchers brought them back to a lab and bred a bunch of them and found a lot of offspring could thrive in the warmer more acidic water, so they wanted to reintroduce them to a reef (or maybe they had). Been a while but it was interesting. Not sure what it takes to shift away from cloning in nature, but waiting for most the reef to die off seems backwards ;-)

One major issue though is diversity. Many coral gardeners use fragments to clone one type over and over.